The energy efficiency of building envelopes (walls, floors, facades) should meet the requirements of regulatory documents. The current focus is mainly on the structural and technological measures aimed at increasing the thermal resistance of protections. Increasing the thermal resistance of wall structures requires estimating the thermal stability of the walls. It has been shown that increased thermal resistance of thermal conductivity does not always provide a steady temperature of the wall’s faces. In this paper, we have found the connection between the active (thermal resistance) and the reactive (accumulation) resistances of an envelope using the model of a one-dimensional wall.